1. Field of the Invention
This invention relates to instruments for measuring concentrations of gas by ultrasonic wave and particularly to a gas-concentration measuring instrument capable of measuring the concentration of gas with high precision for a long continuous period of time by using a highly dampproof ultrasonic sensor.
2. Description of the Prior Art
There is known an instrument for measuring the concentration of a mixed gas or a single-component gas by utilizing the dependency of the propagation speed of ultrasonic wave on the concentration of gas to be measured as disclosed in, for example, U.S. Pat. No. 4,220,040 issued on Sept. 2, 1980 to the same assignee as the present application. The principle of the measurement will first be described.
The propagation speed of ultrasonic wave in a mixture gas is determined by the constants, concentration and temperature of the mixture gas. In other words, the propagation speed can be expressed by the following equation (1) ##EQU1## where v: propagation speed of ultrasonic wave in the mixture gas,
c.sub.pi : specific heat of the object gas i at constant pressure in the mixture gas, PA1 c.sub.vi : specific heat at constant volume of the object gas i in the mixture gas, PA1 M.sub.i : molecular weight of the object gas i in the mixture gas PA1 X.sub.i : mole fraction of the object gas i of the PA1 R: gas constant, and PA1 T: absolute temperature of the mixture gas.
mixture gas,
If the mixture gas is assumed to be comprised of air and carbon dioxide CO.sub.2, Equation (1) is rewritten as EQU v.sup.2 =(C.sub.pco.sbsb.2 X.sub.co.sbsb.2 +C.sub.pair X.sub.air)/ (C.sub.vco.sbsb.2 X.sub.co2 +C.sub.vair X.sub.air).multidot.R.multidot.T(2)
The propagation speed of ultrasonic wave was calculated at each concentration of carbon dioxide CO.sub.2 by substituting the constants and the absolute temperature, 293.degree. K. of the mixture gas into Equation (2). The results are shown in Table 1 and FIG. 1.
TABLE 1 ______________________________________ CO.sub.2 wt % 0 20 40 60 80 100 ______________________________________ Mole rate 0 0.141 0.305 0.503 0.725 1.00 X.sub.CO.sbsb.2 v m/sec 343.07 329.66 315.56 300.30 285.04 268.25 ______________________________________
Since .SIGMA.X.sub.i =1, Equation (2) is alternatively given as the following equation (3): ##EQU2## Thus, the concentation X.sub.CO.sbsb.2 is given as the following equation (4): EQU X.sub.CO.sbsb.2 =F(v, T) (4)
In other words, the concentration of the object gas is the function of the propagation speed of ultrasonic wave v and the gas temperature T.
FIG. 2 shows a block diagram of a measuring system according to the present invention, designed on the basis of the above-mentioned theory.
Referring to FIG. 2, an ultrasonic sensor 1 includes a transmitting transducer 2 and receiving transducer 3 disposed opposedly to the transmitting transducer 2. This ultrasonic sensor 1 is mounted within an object gas atmosphere 4 by a proper method. The ultrasonic wave transmitted from transmitting transducer 2 is passed through the ultrasonic wave path 5 containing the object gas and received by the receiving transducer 3. The speed at which the ultrasonic wave passes through the ultrasonic wave path 5 is inversely proportional to the concentration of the object gas. The transmitting transducer 2 comprises an electrostrictive element. A drive amplifier 6 and a negative immitance converter 7 are used to amplify a high-frequency signal generated from a signal generator 8 that is controlled by a feedback oscillation amplifier 10 and to improve the response characteristic. The receiving transducer 3 comprises an electrostrictive element. A preamplifier 9 is used to amplify the high-frequency signal from the receiving transducer 3 and supplies its output to the feedback oscillation amplifier 10. The resistor 11 and the negative immitance converter 12 are used to improve the response characteristic and the sensitivity of the receiving transducer 3.
On the other hand, the frequency, fm of the above-mentioned feedback oscillating system 13 has a relationship with the propagation speed v of the ultrasonic wave that passes through the path 5 within the object mixture gas, i.e., f.sub.m =k.multidot.v/l (where l is the distance between the transmitting transducer 2 and the receiving transducer 3 and k is a constant of proportionality). Thus, the frequency f.sub.m of the feedback oscillating system 13 and the stable reference frequency f.sub.o generated from the crystal oscillator element 14 are applied to the mixer 15 where the difference F between f.sub.m and f.sub.o is determined. This value F is converted into a voltage by the frequency-voltage converter 16 and supplied to the compensator 17.
A temperature sensor 18 comprises a thermistor, a temperature measuring resistor or a kind of thermocouple for measuring the temperature of the object gas atmosphere 4. The resulting temperature data is supplied to the compensator 17 for temperature by which the temperature dependency of the propagation speed of ultrasonic wave is eliminated. The temperature-compensated output voltage is indicated on the display unit 19 comprising an analog voltmeter, a digital voltmeter or a recorder.
An example of the measuring method of CO.sub.2 gas concentration in a mixture gas comprising three components of air carbon dioxide CO.sub.2 and water vapor H.sub.2 O according to the present invention will be described in detail with reference to FIGS. 2 and 3.
The gas cylinder 20 containing 100% CO.sub.2 gas and a compressor-type air pump 21 respectively supply CO.sub.2 gas and air to flow meters 22 and 23 with flow-adjusting valves by which the concentration of CO.sub.2 gas is adjusted in advance. A mixing chamber 24 for mixing CO.sub.2 gas and air is provided after the flow meters 22 and 23. The CO.sub.2 /air mixture gas from the mixing chamber 24 is introduced through a lead tube 26 into a measuring chamber 25. A water bath 27 sufficiently deep to immerse the lead tube is provided on the bottom of the measuring chamber 25. The CO.sub.2 /air mixture gas is blown off from gas blow-off holes provided appropriately in the lead tube 26 through the water bath 27 into the measuring chamber 25. By doing so, the relative humidity in the measuring chamber 25 increases to as high as 95 to 100%. On the upper region of the measuring chamber 25 there is provided a stirring fan 29 which is rotated by a motor 28. This stirring fan 29 serves to make the concentration of the mixture gas in the measuring chamber 25 uniform. The mixture gas is exhausted through a mixture gas outlet pipe 30 to the outside of the measuring chamber 25. The ultrasonic sensor 1 according to the present invention is disposed at an appropriate position in the measuring chamber 25, and connected by a shielded cable 31 to a computation control section 32 which includes the feedback oscillating system 13, the crystal oscillator element 14, the mixer 15, the frequency-voltage converter 16 and the compensator 17. The temperature-compensating temperature sensor 18 including a temperature-measuring resistor is connected through a cable 33 to the compensator 17 of the computation control section 32. The output voltage from the compensator 17 is set at 0 to 20 V against the CO.sub.2 gas concentration of 0 to 20% by volume, so that the reading of the output voltage represents the concentration of the CO.sub.2 gas. As the display unit 19, a digital voltmeter is used, and the frequency f.sub.m of the feedback oscillating system 13 is monitored by a frequency counter 34. The mixture gas led out through the mixture gas outlet pipe 30 is introduced through an exhaust pipe 36 into an infrared gas analyzer 35 by which the CO.sub.2 gas concentration is measured. In addition, a sampling port 37 for the gas chromatograph is provided on the way of the exhaust pipe 36, so that the CO.sub.2 gas concentration is checked by the gas chromatograph.
A thermister temperature sensor 39 for measuring the temperature of the mixture gas is provided in the measuring chamber 25, which temperature is monitored by a temperature measuring instrument 40. The measuring chamber 25 is completely sealed except for the mixture gas inlet pipe 41 and the mixture gas outlet pipe 30. Moreover, the measuring chamber 25 is placed within a temperature-variable air constant-temperature oven 42 which can be controlled to within .+-.0.1.degree. C. in order that the temperature within the measuring chamber 25 can be arbitrarily changed.
Thus, on this ultrasonic gas concentration measuring instrument, CO.sub.2 gas concentration values changed in the range of 0 to 20% by the flow meters 22 and 23 were actually measured for different temperatures of 27.degree. C., 35.degree. C. and 42.degree. C. within the measuring chamber 25, and the measured data from the infrared ray gas analyzer 35 and the gas chromatograph 38 are shown in Table 2 and FIG. 4.
TABLE 2 __________________________________________________________________________ Ultrasonic Ultrasonic Temp. Gas concentration concentration Infrared ray within Flow meter, set chromatograph meter meter gas analyzer chamber concentration concentration frequency*.sup.1 concentration*.sup.2 concentration (.degree.C.) (Vol %) (Vol %) f.sub.m (Hz) (Vol %) (Vol %) __________________________________________________________________________ 27.0 0 0 37.200 0 0 5 5.58 36.752 5.6 5.6 10 10.36 36.370 10.4 10.5 14 14.14 36.075 14.1 14.1 17 17.16 35.835 17.1 17.2 35.0 0 0 37.536 0 0 4 4.07 37.205 4.1 4.2 11 11.04 36.652 11.0 11.2 12 12.48 36.535 12.5 12.8 17 16.63 36.209 16.6 16.9 42.0 0 0 37.870 0 0 3 3.37 37.596 3.4 3.4 8 7.62 37.265 7.6 7.9 12 12.19 36.890 12.2 12.6 16 15.84 36.606 15.8 16.4 __________________________________________________________________________ *.sup.1 Frequency of ultrasonic wave gas concentration measuring system. *.sup.2 Reading on ultrasonic wave gas concentration measuring system.
From Table 2 and FIG. 4, it is seen that the frequency fm of the feedback oscillating system of the ultrasonic wave gas concentration measuring system according to the present invention represents a linear characteristic against the concentration indicated by the gas chromatograph and the infrared ray gas analyzer, and that the concentration indicated by the ultrasonic wave gas concentration measuring system is sufficiently identical to the concentration indicated by the gas chromatograph and the infrared ray gas analyzer.
Although the foregoing description concerns an embodiment of the method and system for measuring the gas concentration according to the present invention including the three gas component of CO.sub.2, air and H.sub.2 O, the present invention is not limited to such a composition of the mixture gas.
The ultrasonic sensors will hereinafter be described which are respectively used in the ultrasonic-wave transmitting element and the ultrasonic-wave receiving element of the ultrasonic wave gas concentration measuring system. The sensors including the elements have the same structure as that of the conventional one. That is, the sensor of this structure is formed by an ultrasonic transducer (for example, piezo-electric ceramic such as PZT) having silver electrodes fused together and which is attached to a plate or holder (made of, for example, metal or plastics). This type of ultrasonic sensor has so far been used for transmission or reception of ultrasonic wave in the measure/control ultrasonic equipment. This type of sensor has a drawback that the electrodes on the surface of the PZT or the like, for example, silver electrodes are easy to be electrically corroded in the presence of water vapor, thus often making it difficult to correctly convert an ultrasonic-wave signal to an electric signal and vice versa. Thus, in order to prevent the electrodes on the ultrasonic transducer from being electrically corroded, a sealing material such as silicone resin, epoxy resin or polyurethane has been utilized for preventing water vapor from entering into the holder. Nevertheless, even the ultrasonic sensor with its ultrasonic transducer attached to the holder and sealed with a sealing material was erroded by gradual intrusion of water vapor after it was continuously operated for as long a time as one to ten years in the atmosphere of 80 to 100% humidity. There is another countermeasure against the electric corrosion which employs a metal holder and seals it by welding. However, the adhesive resin with which the ultrasonic transducer is attached to the metal holder is easy to be deteriorated by heat upon welding.
The prior art will be described in more detail with reference to the accompanying drawings. FIG. 8 shows an example of the conventional ultrasonic sensor. FIG. 8a is a cross-sectional view of an ultrasonic sensor 59 and FIG. 8b is an enlarged cross-sectional view of an ultrasonic transducer 45 (made of, for example, a piezo-electric ceramic material such as PZT, or a resin material having a piezo-electric characteristic) and its peripheral portion.
As shown in FIG. 8b, the ultrasonic transducer 45 is attached with electrodes 60 and 61 and bonded to a holder 44 (made of for example, metal or resin) with an adhesive agent 62 having a good characteristic for propagation of the ultrasonic wave. The electrodes 60 and 61 are connected through wires 46 and 47 to terminals 48 and 49 which are fastened to a base 50 (made of for example, phenol resin laminated board or epoxy laminated board) as shown in FIGS. 8a and 8b. The holder 44 is sealed by covering the base with a sealing material 51. This structure, however, has a drawback that when it is continuously operated for as long a time as, for example, one year to 10 years in the atmosphere including water vapor, water vapor enters into the holder 44 through the sealing material 51, making the electrodes 60 and 61 be electrically corroded so that the ultrasonic wave cannot be correctly transmitted and received.